The present invention relates to a method for the thermal treatment of substrates, according to which the substrate is heated by a plurality of separately controllable heating elements.
Such apparatus are used, for example, in the semiconductor industry subsequent to lacquering processes of substrates, especially photomasks, in order to thermally treat the substrates for curing and chemical pretreatment of the layers. During the thermal treatment, it is important for the subsequent usability of the substrates that the applied layers be treated as uniformly and homogeneously as possible. In this connection, however, the problem occurs that during uniform control of the heating elements the substrates cannot be uniformly treated due to edge effects. It is therefore known to heat the heating elements less strongly in the edge region of the substrates at the beginning of the thermal treatment, since in the central portion initially a relatively greater mass must be heated. The heating elements are later heated more strongly in the edge portion of the substrates due to the increased thermal irradiation in these regions.
DE-A-199 07 497, which originates with this same applicant discloses, an apparatus and a method for the thermal treatment of substrates, according to which a plurality of separately controllable heating elements first heat a heating plate and then a substrate disposed thereon. The individual heating elements are respectively regulated by a PID regulator, whereby the necessary desired value is prescribed in the form of a desired-value profile. In this context, desired-value profile is understood to mean the determination of desired values that vary with time, especially in time intervals.
During a treatment process, the temperature distribution on the substrate surface facing away from the heating plate is determined and, as a function of the temperature distribution on the substrate surface relative to the desired-value profile, changed desired values are defined for the individual heating elements and are conveyed on to the respective PID regulator. This provides the possibility of undertaking corrections of the desired-value profiles during the treatment process in order to achieve an improved temperature homogeneity over the substrate surface.
However, this method has the problem that it can always react only to already-occurring temperature inhomogeneities on the substrate surface in order to subsequently correct them via a change of the desired-value profile of the individual heating elements. An anticipation of future temperature inhomogeneities that occur during the treatment is not possible. In addition, the problems that occur during one treatment, for example an increased temperature inhomogeneity at a specific temperature value, are not taken into account during the treatment of the next substrate, since the previously established desired-value profile is again prescribed for the individual heating elements. One can thus expect the same problems from the previous treatment to occur during the next treatment.
Proceeding from this state of the art, is therefore an object of the present invention to provide a method for the thermal treatment of substrates that enables a self-optimizing process control in order to increase the temperature homogeneity on the surface of a substrate that is to be treated.